Ignition mixtures and electric initiators



Oct. 13, 1959 D. T. ZEBREE mumou MIXTURES AND ELECTRIC mmmons FiledMarch 30, 1956 DAVID T. ZE BREE INVENTOR.

United States Patent Ofilice IGNITION MIXTURES AND ELECTRIC INITIATORSDavid T. Zebree, Kingston, N.Y., assignor to Hercules Powder Company,Wilmington, Del., a corporation of Delaware Application March 30, 1956,Serial No. 575,026 19 Claims. 01. 52-21 tion is a continuation-in-partof my copendingapplication, Serial Number 78,383, filed February 25,1949, now abandoned.

In the maufacture of delay electric initiating devices one of the mostcommonly used and most satisfactory igniton compositions is a mixture offinely-divided lead and selenium. These mixtures are substantiallygasless during combustion, and, for the most part, have proved mostsatisfactory in igniting fuse elements containing delay trains composedof such difficultly ignitable mixtures as barium peroxide-selenium.

A very substantial problem has been encountered, however, due to thedeterioration is quality of these mixtures which results from theoxidation of the lead. This deterioration is most pronounced during warmweather storage and its rate is accentuated by the presence of moisture.Such deterioration has two very undesirable effects on the performanceof a delay electric blasting initiator which flow directly from thedimunition in calorific content of the mixture due to the progressiveoxidation.

First, irregularity in firing times results with a consequentoverlapping among delay periods in series firing. Second, actual fuseignition failures result in numbers directly proportional to the degreeof oxidation of the lead which has taken place. As a result of suchdeterioration, it has been necessary to purchase special and moreexpensive grades of lead powder which have a very low initial oxidecontent. It is then necessary to exercise meticulous care in thehandling and storage of both the ignition mixture and the completedinitiating devices. Despite all such precautions, however, a progressivedeterioration in quality may still take place in time, with theundesirable conseqences above set forth.

A further problem has been encountered when delay electric blastinginitiating devices containing lead-selenium mixtures are fired in aninverted position. Even when the ignition mixtures contain noappreciable oxide content, occasional failures are still encounteredduring inverted firing due to the tendency of the 'leadfselenium mixtureto shrink during combustion. The failures are due to an actual shrinkingaway from the delay fuse to the extent that the ignited ignition powderdoes not make sufiicient contact 'for ignition of the delay train.

Therefore, an object of this invention is a delay electric initiatingdevice of greatly improved performance which is not subject to failuredue to deterioration of its ignition mixture. Another object of thisinvention is a delay electric initiating device comprising an ignitionmixture containing lead and selenium which is not subject toirregularity in firing time due to improper ignition. Another object ofthis invention is a delay electric initiating device Patented Oct. 13,1959 2 comprising an ignition mixture containing lead and selenium whichis not subject to ignition failures during inverted firing. Stillanother object is to provide leadselenium ignition mixtures exhibitingimproved ignition characteristics. Yet another object is to provide amethod for efiectingignition in the detonation of an explosive employinga primer. Other aspects and objects will 'be apparent from theaccompanying disclosure and the appended claims.

In accordance with this invention an ignition mixture is provided whichcomprises lead and selenium in ignitable proportions, as the majoringredients, and an amount of silicon capable of imparting increasedheat of combustion when said lead and selenium are burned. Still inaccordance with this invention is provided an electric initiating devicehaving in combination a delay fuse element and a loose ignition .mixturein contact with the delay fuse element, said mixture comprising lead'and sele-' nium as the major ingredients and an amount of siliconcapable of imparting increased heat of combustion when said lead andselenium are burned. Further, in accordance with this invention isprovided a method for effecting detonation of a primer composition in ablasting cap, comprising placing an ignition mixture in communicationwith said primer, in said cap, so as to effect detonation of said primerresponsive to heat formed when said mixture is burned, said ignitionmixture comprising lead and selenium in ignitable proportions, as themajor ingredients, and an amount ofsilicon capable of impartingincreased heat of combustion when said lead and selenium are burned, andigniting said mixture.

Although it is within the scope of my invention to utilize anyignitableproportions of lead and selenium, i.e., capable of undergoingcombustion when cont-acted with a heated bridge wire as discussedhereinafter, it is now preferred for optimum results that the lead andselenium be present in the mixture in substantially stoichiornetricproportions. Such a chemically equivalent mixture contains, by weight,72.4% lead and 27.6% selenium. However, mixtures containing any suitableproportions generally between about 64% and about lead and between 20%and 36% selenium can be employed.

Any amount of silicon can be employed in the practice of this inventionwhich is capable of increasing heat of combustion when thelead andselenium are burned. Generally such amount is within the range of about0.3 to 20 Wt. percent of the ignition mixture, from about 1 to 5 percentbeing now preferred. The upper limit of silicon concentration will beset by its etfect on the sensitivity of the ignition mixture, i.e., byits tendency todesensitize the mixture, which may take place atconcentrations some- ,what above 20 percent, dependent upon the specificleadselemum proportions utilized.

Having generally described the present invention, a more specificdescription is presented with reference to the accompanying drawingwhich represents a sectional view of a delay electric blasting cap inaccordance with one embodiment of the invention. t

The drawing represents a ventless delay electric blasting cap comprisinga-bronze shell 1 into the bottom of nium as the iiuse powder 4compressed to the proper density in a lead tube 5. A loose charge ofignition mixture '6 is superimposed on the'fii se element; The'ignitionmixtureis composed'of finely-divided lead and selenium .(72. 4-27.6)/silicon in a ratio of 98/2. A bridge wire-Tisaflixed to'the terminalends of a pair of lead illustration.

' Having now described ,and illustrated the invention,

the followin'giexamplesare given for purposes of further ExampleVentlessdelay electric blasting: initiators were constructed similar tothat shown in the drawing. More specifically, the shell was-of bronzewith a 0.259 inch inside diameter. .A base charge of 0.40 g. ofpentaerythritol tetram'trate was pressed to a height of inch. A

priming chargeof 0.30 g. 'diazodinit'rophenol/potassium chlorate'(75725)was pressed on top of the base charge to a height of inch.' The fuse, 4inch in length, consisted of a lead tube filled with a mixture of bariumperoxide-selenium 'in the ratio of 84/ 16.. The superimposed ignitioncompound comprised lead-selenium in stoichiometric proportions(72.427.6%, respectively)/ silicon, 98/ 2.- This ignition compoundoccupied a space of .inch. sulfur ignition plug inch long was placedover the ignition powder in such a manner that .the leg'wire ends, whichwere connected by a bridge wire, extended into the. loose powder to adistance of inch. A charge of waterproofing was placed on top ofthe-plug and occupied a space of inch. A -inch column of sulfurcompleted the cap.

One hundred eighty such caps were made and 180 others were made whichwere similar in all respects except that the ignition charge waslead-selenium in stoichiometric proportions. In all 360 caps a leadpowder was used which had a lead oxide content of 41%. When these capswere fired, 8 of those with lead-selenium ignition mixtures failed tofire. 7 All 180 caps having leadselenium-silicon ignition mixtures firednormally after the predetermined period of delay.

Example 2 Ventless delay electric blasting caps were made similar tothose of Example 1. Twohundred caps had leadselenium ignition chargeswhile another 200 caps had the lead-selenium-silicon ignition charges.In all 400, the lead powder used was of top quality with a very lowoxide content. When these 400 caps were fired in an inverted position, 2vof those having lead-selenium ignition charges, failed, while all thosehaving lead-seleniumsilicon charges fired normally. In'the two failures,the fused ignition mixture was found to have shrunk away from the delayfuse. I Y

' Example 3 v In the tabulation below are recorded the results ofincorporation of 1 percent-and 5 percent of silicon in a lead-seleniummixture; A lead powder was employed which had an oxidecontent-of 41%.This oxidized powder was mixed with selenium in stoichiometricproportions (72.4% lead to 27.6% selenium) as if no oxidation had takenplace. The delay electric blasting caps employed were those described inExample 1. Corresponding data for lead-seleniumaloneare also set forth.In each test, 25 such. caps were fired in series with a firing currentof one ampere.

1 The ignition powder burned but failed to generate enough heat to andselenium, results in greatly enhanced performance. These results arebelieved due to the great afii nity that silicon has for lead oxide andto the fact its addition causes the burning ignition'mixture to expandrather than contract. While the reaction between lead oxide and siliconin the presence of lead-selenium is not violent, it is exothermic. Whensilicon as a corrective material is used in accordance with thisinvention, there is no adverse eifect on the performance of the ignitionmixture despite 'the' 1 degree to which deterioration in quality of thelead-selenium mixture has taken place. To the contrary, as has beenshown, the presence of as little as 2% of silicon has been found tocompletely compensate for the presence of as much as 41% of lead oxide.

For example, a marked improvement in lead-selenium mixtures is notedwhen 0.3% silicon is incorporated. When 1% silicon is incorporated, fuseignition failures in both upright and inverted firing are eliminated.When 2% of silicon is incorporated, all aberration in firing timesiseliminated. Consequently, while silicon may be incorporated up to 20%and higher without adversely affecting ignition of the delay fuse orfiring time of the cap, 2%.and up to 5% of silicon is generallysufiicient to compensate for the formation of up to 41% of lead oxideand any contraction of the lead-selenium mixture during combustion.

While any of the usual types of delay fuse may be employed in the delayinitiator embodiment of this invention, the barium peroxide-seleniumtype of fuse employed in the examples is preferred. Any of the normalbase-charges and priming explosives are also operable althoughpentaerythritol tetranitrate-base charges anddiazodinitrophenol/potassium chlorate priming charges are preferred. Theusual types of shell, plug, leg'wires, and sealing compound may be usedalthough those employed and illustrated herein are preferred.

Deterioration of lead-selenium ignition mixtures due to the oxidation ofthe lead, therefore, no longer constitutes a problem in the improvedelectric initiators of this invention. 'When the prescribed amounts ofsilicon as the corrective material are incorporated in the ignitionmixture as described and illustrated, it is no longer necessary toselect highest grade load powders or to exercise meticulous care inhandling or storing the ignition mixtures themselves or the completedinitiators. Furthermore, the failures of delay electric initiatorscontaining leadselenium ignition mixtures during inverted firing arealso elminated. Delay electric initiators made in accordance with thepresent invention are therefore easier and less expensive to manufactureand store and exhibit greatly improved performance.

A feature of this invention is in the broad rangeof proportions ofsilicon in my ignition compositions. Thus, when employing a metal suchas aluminum or magnesiumv as a corrective material in a lead-seleniumignition mixture to counteract presence of lead oxide therein, there isa critical relatively low maximum limit of concentration, i.e., 3%,above which a violent burnmg reaction takes place with development offlame which generally results in bursting of the shell and concomitantfailure. Therefore, when employing aluminum or magnesium or other suchflame-producing metals, there is always the inherent danger that theinitiator will inadvertently contain more than the maximum allowa'blemetal'andreact violently, when fired, with concomitant failure.Furthermore, manufacture of such ignition mixtures (containing aluminumor magnesium or the like) is hazardous and requires highly preciseprocedures and equipment for handling such materials particularly toassure that the said critical limit is not exceeded. On the other hand,although silicon present in a lead-selenium ignition mixture in aconcentration of say 2-3 percent often provides an optimum amount ofheat liberation, no violent burning reaction takes place when suchconcentration is exceededeven to as high as 20 weight percent orhigherprecluding thereby any possibility that bursting of theshell willvtake place.

The initiator of this invention is therefore, in all events, free fromany violent burning of the ignition mixture and concomitant bursting ofthe shell to assure against failures. Indeed, if desired, relativelyapproximate amounts of silicon can be utilized in the said ignitionmixture without the need for precise measuring and handling.

Thus, as seen in the following tabulation, aluminum employed in aconcentration of 3% in a lead-selenium ignition mixture functionswithout setting up a violent reaction. However, when 3.5%, and 4%aluminum are employed, failure can take place as result of the violentreaction with bursting of the cap shell.

Ignition powder burned but failed to generate suflicient heat to ignitethe fuse.

I Ignition powder burned so violently that the shell was forciblyruptured and the delay fuse was not ignited.

With reference to Example 3 hereinabove, it is seen that concentrationsof silicon as high as 5% gave no violent reaction. Similarly, siliconconcentrations as high as 20% and higher can be employed as describedhereinabove, in no event there occurring failure resulting from violentreaction involving silicon.

The foregoing clearly illustrates the nonequivalency of silicon withflame-producing metals such as aluminum and magnesium in ignitionmixtures of this invention.

In one embodiment of the method of this invention, an ignition mixturecomprising lead, selenium and silicon as described herein is placed in adelay cap superposed on a suitable delay fuse such as bariumperoxideselcnium. The fuse is superposed on a suitable primer such asdiazodinitropheuol-potassium chlorate, the latter superposed on a highcrystalline explosive such as PETN. A resistance or bridge wire isdisposed in the ignition mixture and is connected by leg wires to anysuitable current source. Current is then passed through the bridge wireand heat is developed responsive to which the primer is detonated, inturn detonating the PETN.

It is to be understood that the initiators of this invention are not tobe limited to delay initiators, but are also inclusive ofinstantaneous-type initiators.

As will be evident to those skilled in the art, various modificationscan be made or followed, in the light of the foregoing disclosure anddiscussion, without departing from the spirit or scope of the disclosureor from the scope of the claims.

What I claim and desire to protect by Letters Patent 1. An ignitionmixture of from 64 to 80 weight percent lead, from 20 to 36 weightpercent selenium and containing an amount of silicon at least sufficientto impart increased heat of combustion when said lead and selenium areburned.

2. An ignition mixture consisting essentially, on a weight basis, offrom 0.3 to 20 percent silicon, from 64 to 80 percent lead and from 20to 36 percent selenium.

3. Anignition mixture of claim 2 containing from 1 to 5 weight percentsilicon.

4. An ignition mixture of lead and selenium, in substantially aleadzselenium Weight ratio of 72.4:27.6 and containing about 2 weightpercent silicon.

5. In an electric initiating device having in combination a delay fuseelement and a loose ignition mixture of lead and selenium in contactwith the said delay fuse element, the improvement providing as saidignition mixture, on a weight basis, from 64-80% lead, from 2036%selenium and from 0.3 to 20% silicon.

6. A method for effecting detonation of a primer composition in ablasting cap comprising placing an ignition mixture of lead, seleniumand silicon in communication with said primer, in said cap, so as toeffect detonation of said primer responsive to heat formed when saidmixture is burned, said ignition mixture containing an amount of siliconat least sufiicient to impart increased heat of combustion when saidlead and selenium are burned, and igniting said mixture.

7. A method of claim 6 wherein said ignition mixture is superposed on adelay fuse, said delay fuse being superposed on said primer.

8. A method of claim 6 wherein said ignition mixture is superposed onsaid primer.

9. A method of claim 6 wherein said mixture is ignited by heat from aresistance Wire in contact therewith, developed by passage of electriccurrent through said Wire.

10. A method of claim 6 wherein said mixture contains from 0.3 to 20weight percent silicon.

11. A method of claim 10 wherein said mixture contains from 1 to 5Weight percent silicon.

12. A method of claim 10 wherein said ignition mixture contains lead andselenium in substantially a lead: selenium weight ratio of 72.4:27.6 andabout 2 weight percent silicon.

13. A method of claim 10 wherein said ignition mixture contains fromabout 64 to weight percent lead, 20 to 36 weight percent selenium.

14. A delay electric initiating device having in combination a delayfuse element and a loose ignition mixture of lead, selenium and siliconin contact with the delay fuse element, the said ignition mixtureconsisting essentially, on a weight basis, of from 64 to 80 percentlead, from 20 to 36 percent selenium and an amount of silicon at leastsuflicient to impart increased heat of combustion when said lead andselenium are burned.

15. An initiating device comprising in combination a loose ignitionmixture of lead, selenium and silicon in communication with a primerwhich detonatcs in response to being heated, so as to cause said primerto detonate in response to heat generated when said ignition mixture isburned, said ignition mixture consisting essentially, on a weight basis,of from 64 to 80 percent lead and from 20 to 36 percent selenium andcontaining an amount of silicon at least sufficient to impart increasedheat of combustion when said lead and selenium are burned.

16. An initiating device of claim 15 wherein said ignition mixture is indirect contact with said primer.

17. A device of claim 15 wherein said ignition mixture contains from 0.3to 20 weight percent silicon.

18. A device of claim 15 wherein said ignition mixture contains from 1to 5 Weight percent silicon.

19. A device of claim 15 wherein said mixture contains lead and seleniumin substantially a lead:selenium weight ratio of 72.4:27.6 and about 2weight percent silicon.

References Cited in the file of this patent UNITED STATES PATENTS

